The present invention is directed generally to the coating arts and more particularly to a novel non-contact coating apparatus for applying a coating to a surface of a moving workpiece.
While the invention may find utility in other applications, the disclosure will be facilitated by reference to the non-contact application of a liquid glue medium to a moving substrate such as a paper carton, in a desired pattern. In the manufacture of folding paper cartons and the like it is desirable to apply glue to predetermined surfaces of the carton and in predetermined patterns for use in later erecting and assembling the carton.
Heretofore, primarily contact methods of gluing have been used in this application. In one contact method, a glue applicator in the form of a roller or like apparatus contacts the surface of each carton as a plurality of cartons move down a conveyor. However, the contact method of glue application has a number of drawbacks, for example, this method is messy and difficult to control.
In many instances, the limitations of the gluing wheel may be circumvented by replacing it with a controllable extruding applicator which rides upon the surface of the substrate and is maintained in contact with the substrate by a spring loading arrangement. Most often, this applicator is pneumatically operated with the pneumatic system being controlled in turn by an electrically actuated pilot valve. By driving the pilot valve from an appropriate electronic controller, it is possible to lay down a desired glue pattern. Such systems have been employed successfully in many applications, most notably the gluing of corrugated cartons. However, their usefulness at high surface speeds is limited by the relatively slow response of the electropneumatic components as well as by the dynamics of the suspension which maintains the contact between the applicator and the substrate. Moreover, this approach is not suitable for use with more delicate media, such as small folding cartons, which are normally run at higher speeds and which are unable to sustain the forces imposed by the spring-loaded applicator. In such situations, attempts to employ a surface-riding applicator are likely to result in damage to the cartons and/or frequent jamming of the cartons in the transport.
As a solution to the foregoing problems, non-contact extrusion gluing systems have been proposed. In such systems, a nozzle-type applicator is generally spaced some distance above the surfaces of the cartons on the conveyor for applying glue in the desired pattern. However, problems have also arisen with the non-contact system. For example, some delay is inherent between the release of glue from the nozzle and its impingement upon the surface of the carton. Hence, proper timing of the opening and closing of a valve feeding the dispensing nozzle is required to compensate for this delay and ensure the proper disposition of the desired pattern upon the carton surface. As the speed of the conveyor increases, this compensation becomes increasingly important.
Moreover, it is a significant problem in non-contact gluing devices to secure an accurate and well-defined termination of a glue bead. This problem is largely one of "tailing" of the glue bead deposited on the carton surface following the shut-off of the dispensing nozzle valve. By "tailing" is meant the tendency of the remaining glue both in the nozzle and in the space between the nozzle and the carton surface to be drawn out into a long, thin tail after the closing of the valve. This is because the closing of the valve results in loss of pressure driving the glue to the carton. Hence, the continued movement of the conveyor, and hence carton, past the nozzle, coupled with the rapidly decreasing velocity of the remaining glue tends to draw out this remaining glue in a relatively long tail on the surface of the carton.
Such a tail is undesirable as it causes glue to be applied to parts of the carton outside of the selected or desired glue pattern. This undesired tailing may take the form of either a continuous tapering trail of glue or may break up into a number of individual globules with spaces therebetween. In any event, extension of the glue bead beyond the desired limits creates serious problems in most applications. Often, it will result in tacking together portions of the carton which should be free of one another or cause the carton to jam in the automatic machinery which handles it for filling. Hence, such tailing is a substantial problem because of the delays and shutdowns it causes later in the automated carton erection and filling operations.
Merely setting the timing of glue valve closing to allow this tail to serve as the final portion of a glue bead or pattern has been found to be unsatisfactory. Firstly, as mentioned, the tail produced is generally inconsistent, and cannot be relied upon to provide a reproducible pattern or bead. Moreover, the end of the bead formed by a tail tends to be lacking in strength because of the diminishing quantity of glue. A carton in which some portions are not fully glued can be equally unacceptable to one in which glue is applied at undesired portions.
This problem of tailing increases with increased velocity of the conveyor carrying the cartons past the glue application nozzle. Additionally, the problem increases as the separation or spacing between the nozzle and the carton surface increases. However, greatly decreasing this latter spacing is not acceptable as it eliminates the desired tolerance for boxes which may be slightly higher or lower upon the conveyor, and thus gives rise to similar problems of jamming encountered in contact gluing applications as described above. Running a gluing line at sufficiently low surface speed to minimize the tailing problems is also unacceptable from a standpoint of productivity.
The presence or absence of a glue tail is also affected by the glue viscosity, the orifice diameter of the nozzle and the pressure of the glue fed to the nozzle. However, control of these parameters is often severely limited by the application. For example, the parameters of viscosity, orifice diameter and glue pressure also affect the size and quality of the glue bead deposited. Hence, it is not realistic to resolve the tailing problem at the cost of the desirable properties of the main glue beads being deposited. Moreover, a relatively small orifice which would tend to minimize the tailing problem would also be highly susceptible to clogging by dirt particles or by the glue itself. Also, a relatively small orifice would result in a relatively fine glue stream which would be more susceptible to external influences such as aerodynamic drag, or the like, especially in the space between the glue nozzle orifice and the surface of a carton. Since the glue orifice, viscosity and pressure are interdependent, the requirements dictating any one of these parameters tends to fix the possible values of the other two. Unfortunately, requirements of a particular application including the production rate, machine tolerances and vibrations, and need for particular glue bead characteristics peclude realization of the necessary combination of parameters to avoid the problem of tailing in most applications.